1. Field of the Invention
The present invention generally relates to a mobile communication system. More particularly, the present invention relates to an apparatus and method for scheduling downlink packets.
2. Description of the Related Art
Mobile communication systems are under development to provide high-speed large-data services beyond voice-focused service. A Code Division Multiple Access 2000 (CDMA 2000) 1xEvolution Data Only (1xEV-DO) system provides high-data services. A Base Station (BS) transmits a downlink signal with a maximum power and a Mobile Station (MS) measures the Carrier-to-Interference (C/I) ratio of the signal and feeds back a Data Rate Control (DRC) value corresponding to the C/I ratio to the BS. The DRC value is a data rate representing the downlink channel status of the MS.
With DRC values received from MSs, the BS selects an MS (or a flow) to which it will transmit a signal and the format of a transmission packet. The transmission packet can be a single-user packet directed to one MS or a multi-user packet directed to a plurality of MSs.
FIG. 1 is a block diagram of an apparatus for transmitting and receiving signals between a BS and an MS in a conventional CDMA 1xEV-DO communication system.
Referring to FIG. 1, upon receipt of a downlink signal from a BS 100, an MS 150 decodes the downlink signal at a decoder 108 and provides the decoded signal to an Automatic Repeat reQuest (ARQ) decider 114 and a target Packet Error Rate (PER) controller 116. The ARQ decider 114 selects an ACKnowledgment/Negative ACKnowledgment (ACK/NACK) signal for the received downlink signal and transmits the ACK/NACK signal to the BS 100. The target PER controller 116 adjusts a DRC decision threshold for a DRC decider 112, taking into account packet error information about the decoded signal, in order to determine a DRC value that satisfies a target PER of 1%.
A C/I estimator 110 estimates the C/I ratio of the downlink signal received from the BS 100. The DRC decider 112 compares the C/I ratio estimate with the DRC decision threshold, determines a DRC value that makes the downlink signal satisfy the target PER of 1%, and transmits the determined DRC value to the BS 100.
The BS 100 decodes DRC values and ACK/NACK signals received from the MS 150 and other MSs at a DRC decoder 104 and an ARQ decoder 106. A scheduler 102 performs a scheduling algorithm according to the decoding results to thereby allocate downlink resources to the MSs.
The BS uses a transmission format corresponding to a DRC value received from an MS, for transmission of a single-user packet to the MS. The preamble of the single-user packet carries information about the MS to receive the single-user packet, and the MS demodulates the packet using the transmission format corresponding to its transmitted DRC value.
The preamble of a multi-user packet provides information indicating whether this packet is a multi-user packet and information indicating a packet size. Information about a plurality of MSs is included in a Medium Access Control (MAC) header of the multi-user packet. These MSs are those that have transmitted compatible multi-user DRC values. The compatible multi-user DRC values refer to DRC values including the same multi-user transmission format in Table 1. A DRC value is a DRC index in Table 1. The compatible multi-user DRC values range from a low DRC value to a high DRC value.
FIG. 2 is a flowchart of a downlink scheduling operation of a BS in a conventional communication system.
Referring to FIG. 2, the BS prioritizes MSs in step 202. For example, for a Best Effort (BE) service, the BS can prioritize the MSs by a proportional fairness algorithm. For a Quality of Service (QoS) sensitive time delay, the BS can prioritize the MSs in a descending order of time delay.
In step 204, the BS generates a plurality of candidate transmission formats for an MS according to the priority level of the MS and selects one of the candidate transmission formats, for a transmission packet. If possible, the BS converts the transmission packet to a short transmission packet in step 206 and transmits the packet in the selected transmission format to the MS in step 208.
Table 1 below lists transmission formats for a CDMA 2000 1xEV-DO Revision A communication system.
TABLE 1List of AssociatedList of AssociatedRateSingle-UserMulti-UserDRCMetricSpanTransmissionTransmissionindex(kbps)(slots)FormatsFormats0x0016(128, 16, 1024)(128, 4, 256)(256, 16, 1024)(256, 4, 256)(512, 16, 1024)(512, 4, 256)(1024, 16, 1024)(1024, 4, 256)0x138.416(128, 16, 1024)(128, 4, 256)(256, 16, 1024)(256, 4, 256)(512, 16, 1024)(512, 4, 256)(1024, 16, 1024)(1024, 4, 256)0x276.88(128, 8, 512)(128, 4, 256)(256, 8, 512)(256, 4, 256)(512, 16, 512)(512, 4, 256)(1024, 8, 512)(1024, 4, 256)0x3153.64(128, 4, 256)(128, 4, 256)(256, 4, 256)(256, 4, 256)(512, 4, 256)(512, 4, 256)(1024, 4, 256)(1024, 4, 256)0x4307.22(128, 2, 128)(128, 4, 256)(256, 2, 128)(256, 4, 256)(512, 2, 128)(512, 4, 256)(1024, 2, 128)(1024, 4, 256)0x5307.24(512, 4, 128)(128, 4, 256)(1024, 4, 128)(256, 4, 256)(2048, 4, 128)(512, 4, 256)(1024, 4, 256)(2048, 4, 128)0x6614.41(128, 1, 64)(128, 4, 256)(256, 1, 64)(256, 4, 256)(512, 1, 64)(512, 4, 256)(1024, 1, 64)(1024, 4, 256)0x7614.42(512, 2, 64)(128, 4, 256)(1024, 2, 64)(256, 4, 256)(2048, 2, 64)(512, 4, 256)(1024, 4, 256)(2048, 4, 128)0x8921.62(1024, 2, 64)(128, 4, 256)(3072, 2, 64)(256, 4, 256)(512, 4, 256)(1024, 4, 256)(2048, 4, 128)(3072, 2, 64)0x91228.81(512, 1, 64)(128, 4, 256)(1024, 1, 64)(256, 4, 256)(2048, 1, 64)(512, 4, 256)(1024, 4, 256)(2048, 4, 128)0xa1228.82(4096, 2, 64)(128, 4, 256)(256, 4, 256)(512, 4, 256)(1024, 4, 256)(2048, 4, 128)(3072, 2, 64)(4096, 2, 64)0xb1843.21(1024, 1, 64)(128, 4, 256)(3072, 1, 64)(256, 4, 256)(512, 4, 256)(1024, 4, 256)(2048, 4, 128)(3072, 2, 64)0xc2457.61(4096, 1, 64)(128, 4, 256)(256, 4, 256)(512, 4, 256)(1024, 4, 256)(2048, 4, 128)(3072, 2, 64)(4096, 2, 64)0xd15362(5120, 2, 64)(128, 4, 256)(256, 4, 256)(512, 4, 256)(1024, 4, 256)(2048, 4, 128)(3072, 2, 64)(4096, 2, 64)(5120, 2, 64)0xe30721(5120, 1, 64)(128, 4, 256)(256, 4, 256)(512, 4, 256)(1024, 4, 256)(2048, 4, 128)(3072, 2, 64)(4096, 2, 64)(5120, 2, 64)
Referring to Table 1, under List of Associated Single-User Transmission Formats, transmission formats written in bold are canonical transmission formats mapped to DRC values, and the other transmission formats are for short transmission packets. Under List of Associated Multi-User Transmission Formats, transmission formats for packet sizes less than 1024 are for short transmission packets. Under List of Associated Single-User Transmission Formats and List of Associated Multi-User Transmission Formats, transmission formats are represented as (a, b, c) where “a” denotes a physical packet size, “b” denotes a slot span, and “c” denotes a preamble length.
As described above, there are no proposed techniques for specifying how a BS scheduler determines a transmission format for a downlink packet. When the BS selects MSs to receive a multi-user packet, it is inefficient for the BS to check over every MS to determine whether its DRC value includes the same multi-user DRC value.